150 Proceedings of Indiana Academy of Science 
turning the grain inside out. Physical examination shows a profound 
change in the texture of the endosperm, the cell walls being destroyed, 
the starch grains exploded, and other characteristics of organic struc- 
ture obliterated; and chemical analysis indicates hydrolysis of most of 
the starch and-.a considerable loss of moisture. 
Two factors here present themselves for explanation: (1) the 
expansive medium acted upon by the heat; and (2) the structure which 
gives force to the explosion by confining the accumulating pressure 
until a limit is reached. 
Most of the investigators of the subject up to the present have 
been physicists and chemists, and they have satisfactorily solved the 
problem involved in the first of these factors, the expansive medium. 
But they have failed to locate the confining structure, because here the 
clue is afforded by the difference between popcorn and kinds of corn 
that do not pop; and this falls within the field of plant morphology. 
The Expansive Medium.—An old idea, that the expansive medium, 
acting as a vehicle for the disruptive force, was a small volume of air 
imprisoned in the middle of the grain, seems long ago to have been 
abandoned for want of evidence. And the more recent one, attributing 
the explosion to a volatile oil, has gone by the same route. The sig- 
nificant changes that occur in popping indicate that the disruptive force 
is distributed throughout the endosperm, while analysis shows that the 
oil content is limited to the embryo of the grain. 
The occurrence of maximum and minimum moisture contents for 
good popping—although the range is wide—and the visible escape of 
steam during popping, indicate that water contained in the very hygro- 
scopic starch grains themselves is the substance that expands and 
causes the explosion. At least a partial hydrolysis of the starch is 
necessary for best results, and this necessitates slow enough applica- 
tion of the heat to permit dextrinization before the explosion occurs. 
Experience has shown that best results are obtained when the popping 
temperature, which is 175° to 200° C., is reached in two to three minutes 
from the initial application of heat. 
The Confining Structure.—The confinement of the increasing pres- 
sure until the instant of explosion was long attributed to the pericarp 
of the grain, but experiments do not bear out this idea. Pieces of 
grains will pop, as will also grains with holes drilled in them, and 
grains with the pericarp removed. 
A microscopic examination of the endosperm of maize shows the 
contents of each cell to consist of numerous starch grains embedded in 
a mass of desiccated colloidal material, the protoplasm of the cell. 
This colloidal material is the seat of all the protein of the endosperm 
except that in the aleurone layer. The flinty or soft texture of the 
endosperm depends upon how completely the colloidal material fills the 
interstices between the starch grains. The moisture in the starch grain 
is changed to steam during the heating process, but the starch is held 
intact by the colloidal matrix until the limit of its capacity is reached. 
Then the explosion of the starch grains of a few cells at the surface, 
where the tissue is flintiest, releases the external pressure on underly- 
ing units, and the whole ten million grains let go simultaneously. The 
